1. Field of the Invention
The present invention relates to a device and method for automatic focusing control for a video camera system or the like.
2. Description of the Prior Art
Various automatic focusing mechanisms and methods have been proposed for use with imaging systems such as video camera systems. One general automatic focusing mechanism is known as a TTL (Through The Lens) system in which a video signal generated by an imaging device such as a CCD (charge-coupled device) or the like is processed by a microcomputer to measure the sharpness of the image on the CCD, and a lens combined with the imaging device is moved on the basis of the measured sharpness until the image is sharply focused on the CCD.
There are two typical automatic focusing variations based on the TTL principle, depending on how optical information from the subject to be imaged is produced. The first automatic focusing scheme is of a passive nature in that information borne by light coming from the subject and entering a master lens is used directly as automatic focusing information. The second automatic focusing configuration is of an active nature in that a light signal is radiated from the video camera system to the subject and information borne by light reflected by the subject is used as automatic focusing information. The active and passive automatic focusing variations operate mostly on the triangulation principles.
Recently, however, a passive automatic focusing process which utilizes the detected contrast of an image for focusing the image has become widely used. The conventional passive automatic focusing process of this type will be described below with reference to FIGS. 1A through 1D, 2, and 3A through 3D of the accompanying drawings.
When the image of a subject 2 formed in a viewfinder 1 of a video camera is focused and hence can clearly be seen by the user of the video camera, as shown in FIG. 1A, a video signal 3 produced by the imaging device of the video camera and passed through a filter has a high contrast formed of high frequency components, as shown by the steep slopes in the waveform of FIG. 1B. When the image of the subject 2 is defocused, as shown in FIG. 1C, the video signal 3 has a low contrast formed of low frequency components, as shown by the gradual curves in the waveform of FIG. 1D. The passive automatic focusing process is based on this principle. The video camera has a zoom lens assembly composed of a focusing lens, a zoom lens, and a master lens. For the detection of the contrast of the image, the focusing lens, indicated at 6 in FIG. 2, is moved a small distance by an automatic focusing (AF) motor 5 which is energized by an AF control circuit 8 that is supplied with a signal to reach a maximum contrast of the video signal 3 which represents the image of the subject 2 formed on a CCD 7.
In order to detect a maximum contrast level on the CCD 7 or a viewfinder field 9, as shown in FIG. 3A, there is established a relatively large image frame 10 substantially centrally located in the viewfinder field 9, the image frame 10 having a size of H/2.times.V/2 (H represents the horizontal dimension of the viewfinder field 9 and V the vertical dimension of the viewfinder field 97. When the image of a subject which is highest in contrast, e.g., a person 2a, within the image frame 10 is focused, contrast data are produced as indicated by a curve 12 in FIG. 3B. If a striped wall 2b, for example, is present in the background behind the person 2a, as shown in FIG. 3A, however, the focus is shifted to the wall 2b and the lens is focused on the background, as indicated by a curve 13 in FIG. 3B.
One solution is to use a smaller image frame 11 (FIG. 3C) within the viewfinder field 9 for focusing the person 2a sharply regardless of the background behind the person 2a. However, when the face 2d of a person is imaged for a close-up, as shown in FIG. 3C, the image frame 11 is positioned on the nose 2c. Since the nose 2c is viewed as a virtually flat image and produces no contrast, no focusing position can be found in the image frame 11. Nor can a focusing position be established when the subject moves as indicated by the arrow 16 in FIG. 3D.
Generally, the image frame to be established for contrast detection in the viewfinder field should be of a compromising size between large and small. With such image frame, however the lens may be focused on the background rather than the subject or the focusing position cannot be located, depending on the subject imaged in the viewfinder field 9.